The present invention is directed to cutting heads for use in flexible line rotary trimmers. Flexible line rotary trimmers are commonly used for cutting vegetation such as grass and weeds, particularly along walks, fences and flower beds and around trees. These devices comprise a rotary driven head that carries one or more lengths of monofilament line mounted within a housing. Extended end portions of each line project from the housings through guides in the side wall of the housing. As the head rotates at high speed, the end portions of the line are caused to project outwardly from the housing by the centrifugal forces acting thereon and function as cutting blades.
The majority of trimmer heads presently in use employ two separate monofilament lines which are wrapped about a common spool mounted within the head housing. The lines project from the spool and housing through diametrically opposed guides in the side wall of the trimmer housing. When the cutting line projecting from the head breaks or becomes overly worn, fresh line must be extended from the head and the old line severed and discarded. In what are commonly termed manual-type heads, the drive motor, which can be gas or electric, is shut down and a line indexing mechanism on the stationary head is actuated to pay out additional line from the head. This is typically accomplished by the mechanism temporarily disengaging the spool from the head and rotating the spool a predetermined amount relative to the head. By simply pulling on the cutting line, a measured length of fresh line is pulled from the spool and the old line severed and discarded. In an effort to reduce the down time for line replacement, heads commonly referred to as “bump-feed” heads were developed in which the indexing mechanism is activated while the head is still rotating. The user simply presses the rotating head against the ground depressing an actuator button on the bottom of the head which temporarily disengages the line carrying spool from the housing, allowing the spool to rotate relative to the housing. The inertia of the line and centripetal force acting on the line extending from the rapidly rotating head (often erroneously referred to as centrifugal force) causes fresh line to be pulled from the spool and extended from the housing. A metering mechanism limits the relative rotation of the spool with respect to the housing and thus limits the length of the fresh lines paid out of the head. A knife guard provided at a desired radial distance from the central axis of rotation severs the worn lines, leaving the desired lengths of fresh line projecting from the head. In an effort to further streamline the line replacement process, “automatic heads” were developed. Such heads typically include a spring biased pawl-type mechanism that reacts to an imbalance in the inwardly and outwardly directed forces acting on the rapidly rotating line that results from a loss of mass in the line extending from the head due to wear or breakage. The activation of the pawl mechanism temporarily disengages the spool from the housing allowing relative rotation therebetween and the paying out of a metered amount of line. Excess material is again automatically severed by a knife guard as with the bump-feed type head.
While this evolution of the flexible line rotary trimmers from the manual head to the automatic head has significantly reduced operator time and effort in replacing worn and broken line, these advancements have met with some resistance. Each of these types of cutting heads require the user to replace the line when the original supply has been consumed. For the casual home user this can present a problem. Depending on the particular head, generally at least two separate lines, several feet in length, must be wrapped about the spool. If this is not done properly, the line may tangle within the head, interfering with the withdrawal of fresh line and requiring disassembly of the head and rewinding of the line. The more complicated the line feed mechanism, the more critical is the proper winding of the line within the head. Proper winding is particularly important in automatic heads where the slightest tangle can interfere with the functioning of the line pay out mechanism. Thus, there remains a need for a flexible line rotary trimmer head which greatly simplifies the task of line replacement for the home user.
Several attempts have been made to provide a more user-friendly rotary cutting head. Such heads typically employ one or more short lengths of a line which are anchored within the head in lieu of the longer lengths of line wrapped about an interior spool. These heads are frequently referred to as fixed line cutting heads. While fixed line cutting heads have eliminated the need to carefully wrap the line about the spools to prevent tangles, many such heads still require disassembly to replace worn or broken line. A fixed line head which does not require disassembly to effect line replacement is found in U.S. Pat. No. 4,062,114, issued to Luick and entitled “VEGETATION CUTTING APPARATUS.” The cutting heads disclosed therein are provided with one of several differently configured channels terminating in a reduced diameter portion proximate the side wall of the head. A short length of flexible cutting line extends through the channel and extends radially from the head. The line is held in place by an enlarged member secured to the inner end of the length of cutting line which is translatable along the channel but cannot pass through the reduced diameter portion thereof. While such a head and line configuration provides for relatively simple line replacement, each length of replacement line must be provided with a suitable enlarged member at one end thereof, significantly increasing the cost of operation. In addition, if the cutting line were to break at the eyelet or outlet end of the line channel, line removal may be difficult.
Other attempts to provide a fixed line cutting head in which the line can be easily and quickly replaced are found in U.S. Pat. Nos. 5,758,424 and 5,887,348, both issued to Fernando and Ignazio Iacona and entitled “HEAD FOR STRING TRIMMER.” U.S. Pat. No. 5,758,424 discloses a trimmer head in which a pair of opposed lengths of cutting line are inserted into the head adjacent spring biased cams. The springs cause the cams to press against the line with sufficient force to hold the line in place when the head is at rest. To provide a greater line securement force during use, the cams are configured such that their centers of gravity are located between the posts on which they are mounted and an opposed wall against which they press the respective lengths of cutting line. The centrifugal force acting on the cams during the rapid rotation of the cutting head causes the cams to pivot outwardly and press tightly against the lengths of cutting line to secure the line to the head during use. While such a head facilitates line replacement in that it allows fresh lengths of replacement line to simply be inserted into the head through opposed openings therein and be held in place by the spring biased cams, the Iacona head is dependent on rotational velocity for the tight securement of the cutting line. Accordingly, when such a head is used in heavy brush, the drag on the line will cause the rotational speed of the head to drop, decreasing the centrifugal force acting on the cams and thus the gripping forces of the cams against the lengths of line. As a result, the line could be pulled from the head by the brush, rendering the head ill-suited for use in heavy brush.
U.S. Pat. No. 5,887,348 discloses a fixed line head which utilizes a spring-biased mechanism to hold the line in place with a constant gripping force. The mechanism, however, extends through the top of the head housing so that it can be manipulated by the user when the head is at rest to release and secure the line. The extension of the securement mechanism beyond the surface of the head renders the mechanism susceptible to becoming entangled during use which would tend to pull the mechanism outwardly from the head and possibly release the cutting line at a relatively high velocity. Thus, while this embodiment of a fixed line head is not dependent on rotational velocity for its gripping force, it also is ill-suited for use in heavy brush.
It would be desirable to provide a cutting head for flexible line trimmers that retains the ease of line removal and replacement exhibited by the Iacona fixed line cutting heads, but is neither reliant on rotational velocity for line securement nor susceptible to the inadvertent release of the cutting line during use under the severest of cutting conditions. The fixed line cutting head of the present invention obtains these results.